1. Field of the Invention
The present invention relates to a fastening structure for use at a low temperature. More particularly, the present invention improves sealing property of a fastening structure used at an atmospheric temperature lower than an atmospheric temperature at which a fastening operation is performed.
2. Description of the Background Art
Conventionally, as shown in FIG. 4A, in order to flow cooling liquid helium (LHe) through a pipe 2B provided in a superconducting wire, a pipe 2A connected to a refrigerator 3 is connected to pipe 2B via an electrically insulating joint 1 (Japanese Patent Laying-Open No. 59-144317). As shown in FIG. 4B, electrically insulating joint 1 is implemented by joining and winding an insulating body 5 composed of GFRP (glass fiber reinforced plastic) on an outer circumference of a metal pipe 4 provided at terminal ends of pipe 2A and pipe 2B at a room temperature with an adhesive. Metal pipe 4 uses a material less heat-shrinkable than GFRP, such as copper Cu, and insulating body 5 and metal pipe 4 undergo heat shrinkage in a direction fastening each other. Therefore, even when metal pipe 4 and insulating body 5 undergo heat shrinkage due to flow of cryogenic liquid helium through an internal passage of metal pipe 4, leakage of liquid helium does not take place.
Meanwhile, Japanese Patent Laying-Open No. 11-336725 proposes a joint structure used for a high-temperature pipe arrangement as shown in FIG. 5, in which a bolt 7 penetrates a pair of flanges 6, nuts 8 are screwed from opposing sides for clamping, and washers 9 having a coefficient of linear expansion larger than bolt 7 and nut 8 are interposed between flange 6 and nut 8. With such a structure, even when the flange 6 is heated to a high temperature and bolt 7 expands, washer 9 expands more than bolt 7. Therefore, lowering of a fastening torque can be prevented.
On the other hand, in electric power equipment such as terminal equipment for superconducting cable as shown in FIG. 4, assembly on site is required because of a large size of the equipment. In such a case, it is difficult to achieve a sealing junction with the adhesive, considering a time for an assembly operation or reliability in the sealing property.
In addition, though the structure shown in FIG. 5 suitably prevents loosening of flange 6 due to temperature increase, it is not effective for a case in which the temperature is lowered and flange 6, bolt 7, nut 8, and washer 9 shrink. In other words, if the structure shown in FIG. 5 is installed at a temperature lower than that at the time of fastening operation, shrinkage of washer 9 is larger than that of bolt 7 and nut 8, because washer 9 has a coefficient of linear expansion larger than that of bolt 7 and nut 8. Consequently, sealing property between nut 8 and flange 6 is impaired. Moreover, the coefficient of linear expansion of flange 6 is not considered with respect to the present structure. Therefore, if flange 6 shrink to a large extent due to lower temperature, sealing property is further impaired.